1. Field of the Invention
The present invention relates to a dynamic damper that is mounted about and fixed onto an outer circumferential surface of a rotative shaft, such as a drive shaft of an automotive vehicle, in order to attenuate vibration excited in the rotative shaft.
2. Description of the Related Art
Generally, such a dynamic damper includes: a cylindrical metallic mass member; a pair of cylindrical elastic fixing portions disposed on the axially opposite sides of the metallic mass member in a coaxial relationship with the metallic mass member with a given axial spacing therebetween; a pair of elastic connecting portions disposed between and connecting the axially opposite end faces of the metallic mass member and opposing axially inner faces of the elastic fixing portions, respectively; and a thin elastic covering portion covering an inner and outer circumferential surface of the metallic mass member. The fixing portions, the connecting portion and the covering portion are integrally formed as a single rubber elastic body member. The dynamic damper, for example, is mounted about a drive shaft of an automotive vehicle with its elastic fixing portions being press-fitted onto an outer circumferential surface of the drive shaft. When vending, torsional, or other undesirable vibrational loads are caused by a rotation of the drive shaft, and are applied to the dynamic damper mounted about the drive shaft as described above, the dynamic damper is able to absorb or attenuate these undesirable vibrational loads with the help of shear deformations or other deformations caused in the elastic connoting portions by resonance of the metallic mass member with respect to these vibrational loads applied thereto.
In order to provide such a dynamic damper at a relatively low cost, it has been attempt to eliminate an adhesive provided between the metallic mass member and a part of the rubber elastic body member, which part is held in contact with the metallic mass member. However, this attempt has resulted in lowering in a capacity of the rubber elastic body member to hold the metallic mass member. For this reason, when the dynamic damper is subjected to vibration, the metallic mass member is likely to rotate relative to the rubber elastic body member, resulting in undesirable variation in resonance action of the dynamic damper. Thus, the dynamic damper may suffer from deterioration in its vibration damping capacity. To cope with these problems derived from the elimination of the adhesive provided between the metallic mass member and the rubber elastic body member, a variety of measures has been proposed. One example of such measures is disclosed in JP-A-2002-98186 (Page 2 and FIGS. 1 and 3), wherein a plurality of slits 2 are formed at respective circumferential positions of both axial end portions of a cylindrical metallic mass member 1. These slits 2 extend through the wall thickness of the metallic mass member 1 with a given axial length, while being open in axial end faces of the metallic mass member 1, respectively. Also, these slits 2 are filled with a rubber-covering layer 3 that covers the inner and outer circumferential surfaces of the metallic mass member 1.
However, the dynamic damper disclosed in the above-mentioned document may suffer from the following problems. Referring to FIG. 12B, a rubber elastic body undergoes shrinkage when being cooled from a relatively high temperature just after a vulcanization process to a room temperature. As a result, a part 3a of the rubber elastic body as well as the rubber-covering layer 3 covering an inner and an outer circumferential surface of the metallic mass member 1 also undergo shrinkage. In this stage, the rubber elastic body should be elastically deformed in directions as indicated by allows in FIG. 12B, so as to compensate decrease in volume of the part 3a filling the slit 2 caused by the shrinkage, by means of displacement of the covering rubber layer 3 toward the slits 2. However, an amount of displacement of the rubber covering layer is prone to be insufficient to compensate the amount of the shrinkage of the part 3a of the rubber elastic body, thus easily causing a gap or crack formed between the slit 2 and the part 3a of the elastic body. In particular, a radially inner part of the rubber covering layer 3, integrally bonded to a radially inner part of the part 3a, also undergoes shrinkage in itself in a radially inward direction. This makes it difficult for the rubber elastic body to make deformation in the direction required to compensate the volume decrease of the part 3a disposed within the slit 2, likely causing a relatively large gap. When subjected to input vibration, the metallic mass member is prone to rotate relative to the rubber elastic body at the part where the gap is formed, resulting in variation in resonance effect of the dynamic damper. Thus, the dynamic damper may possibly suffer from deterioration in its damping capability. Further, the above-described elastic deformation of the rubber elastic body causes tensile stress or other residual stress in the part 3a disposed within the slit 2 as well as the rubber covering layer 3, possible causing deterioration in durability of the dynamic damper. Furthermore, the slits 2 of the metallic mass member 1 make acute edges, and the edges of the slits 2 repeatedly come into contact with the part 3a of the rubber elastic body that suffers from the residual stress in a tensile direction. Therefore, the rubber covering layer 3a is readily to be damaged, possible causing a difficulty in achieving sufficiently a desired durability of the rubber covering layer 3.
In the light of a design of the dynamic damper, if the metallic mass member is made small in its axial length, the width of the slit is prone to be made small in order to avoid a shortage of the mass due to a wide slit. The metallic mass member is generally formed by forging or sintering of metal, since it is cheap to manufacture. Preferably employed is the method of sintering, since it is simple in manufacturing facility. In the case where the metallic mass member is formed by sintering of metal, the provision of the narrow slits makes it difficult to assure strength of a mold at a part where is shaped to form the narrow slits. It is yet more difficult for forging to form the metallic mass member with the narrow slits. On the other hand, the provision of the wide slits makes it difficult to obtain a sufficient mass of the metallic mass member, inevitably expanding the profile of the metallic mass member, whereby the dynamic damper undesirably needs a large space for installation.